Exhaust system

ABSTRACT

An exhaust system ( 1 ) is provided with an exhaust passage that allows exhaust gas discharged from an internal combustion engine to pass therethrough, an exhaust emission control unit including a catalyst ( 2,3 ) such as a three-way catalyst ( 27 ) to purify the exhaust gas, and an exhaust heat collecting unit that converts thermal energy of the exhaust gas into electric energy. The exhaust passage ( 25 ) is formed in the center of the exhaust system provided with the exhaust emission control unit. By-pass passages ( 26 ) are formed at both sides of the exhaust passage ( 25 ) through which the exhaust gas flows without passing through the exhaust emission control unit. The exhaust heat collecting unit ( 29 ) is provided on the outer side of each of the by-pass passages ( 26 ).

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to an exhaust system that purifies exhaust gasusing catalyst and converts thermal energy of the exhaust gas intoelectric energy.

2. Description of Related Art

Generally, an exhaust system includes an exhaust emission catalyst suchas a three-way catalyst to purify exhaust gas discharged from an engineby removing hazardous substance contained in the exhaust gas, forexample, carbon monoxide, hydrocarbon, nitrogen oxides and the like. Thecatalyst becomes effective to purify the exhaust gas when it isactivated at its activated temperature in the range between 350° C. and800° C., for example.

Substantially high thermal energy of the exhaust gas at a hightemperature is partially used for increasing the temperature of theexhaust emission catalyst until it reaches the activated temperature.The rest of the thermal energy of the exhaust gas, however, is dispersedwithout being collected. An exhaust heat power generation apparatus hasbeen developed to collect the thermal energy through conversion thereofinto electric energy.

In a certain type of the aforementioned exhaust heat power generationapparatus, a thermoelectric converting module is interposed between anexhaust pipe (high temperature side) through which the exhaust gas flowsand a cooling unit (low temperature side), and each thermoelectricconverting element of the thermoelectric converting module generatespower in accordance with the temperature difference between the hightemperature side and the low temperature side (Related Art 1:JP-A-10-234194). The temperature difference has to be increased whileraising the temperature at the high temperature side so as to improvethe thermoelectric conversion efficiency. In the other type of theexhaust heat power generation apparatus, the catalyst provided in anexhaust passage is used for purifying the exhaust gas as well asincreasing the exhaust gas temperature (the temperature at the hightemperature side of the thermoelectric converting module) under thereaction heat. Under high load operation the exhaust gas flow from theengine can be split off and a part of it redirected through a by-passwhich contains another catalyst (Related Art 2: JP-A-2000-352313). Inorder to protect the thermoelectric converting-element from exceedingits heat resisting temperature, e.g. during high load operation, theexhaust gas flow can also be redirected via aby-pass (Related Art 3:JP-06-081639).

Generally, in the exhaust system, when the catalytic temperature is low,for example, upon start-up of an engine, it has to be rapidly increasedfor smooth operation of the engine. Accordingly the exhaust emissioncatalyst is provided at a position in the exhaust system where theexhaust gas at high temperature (with high thermal energy) passes, forexample, in the vicinity of an exhaust manifold and the like. Then anexhaust heat power generation apparatus is provided downstream of theexhaust emission catalyst in the exhaust system, for example, at aposition near a sub-muffler. The temperature of the exhaust gas passingat the position downstream of the exhaust catalyst, however, becomes lowbecause it has been used for increasing the catalytic temperature or ithas been dispersed as it flows, resulting in decrease in the thermalenergy. As a result, the thermoelectric conversion efficiency of theexhaust heat power generation apparatus is decreased, failing toeffectively collect the thermal energy.

Under a high load of the engine (at a high engine speed), the exhaustemission catalytic temperature becomes considerably high as it is heatedby the exhaust gas at high temperature in the vicinity of the exhaustmanifold. When the catalytic temperature exceeds the activatedtemperature, its purification effect is deteriorated, which maythermally degrade the catalyst. Conventionally in the exhaust system,when the catalytic temperature exceeds the activated temperature, theengine is operated in a fuel-rich state so as to decrease the catalytictemperature. This may increase the fuel supply quantity, resulting indeteriorated fuel efficiency.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an exhaust system thatprevents degradation of the catalyst while improving the fuelefficiency.

The exhaust system according to one aspect of the invention includes aprimary exhaust emission control unit for purifying the exhaust gas anda first exhaust heat collecting unit for collecting the thermal energythrough conversion thereof into electric energy. The exhaust systemincludes a second passage through which the exhaust gas flows withoutpassing through the primary exhaust emission control unit in addition tothe first passage that allows the exhaust gas to flow therethrough. Theexhaust system includes the primary exhaust emission control unit withinthe first passage so as to purfy the exhaust gas flowing therethrough.The exhaust system includes a control member that serves to change aflow of gas between the first passage and the second passage. In thecase where the catalytic temperature becomes high, the exhaust gas iscontrolled to flow through the second passage in the exhaust system soas to avoid excessive increase in the catalytic temperature owing to theexhaust gas at high temperature. This makes it possible to preventthermal degradation in the catalyst. This may also eliminate the needfor preventing the rise in the catalytic temperature by operating theengine in a fuel-rich state to decrease the exhaust gas temperature.Furthermore, the thermal energy of the exhaust gas can be collected aselectric energy, resulting in improved fuel efficiency.

The exhaust system further may include a secondary exhaust emissioncontrol unit provided on an exhaust passage where the first passage andthe second passage are joined.

In the case where the secondary exhaust emission control unit is allowedto purify the exhaust gas in the exhaust system, the control memberserves to control the flow of the exhaust gas into the second passage.If the exhaust gas can be purified by the secondary exhaust emissioncontrol unit, the first exhaust heat collecting unit is capable ofcollecting the thermal energy from the exhaust gas, resulting inimproved fuel efficiency.

In the exhaust system, the operation of the control member may becontrolled based on the temperature in the primary exhaust emissioncontrol unit or the temperature in the secondary exhaust emissioncontrol unit. The control member is operated such that the exhaust gasflows through the second passage when the temperature either in theprimary exhaust emission control unit or the secondary exhaust emissioncontrol unit exceeds a predetermined temperature. The predeterminedtemperature is determined based on an activated temperature of thecatalyst in the primary exhaust emission control unit or the secondaryexhaust emission control unit.

In the exhaust system, the operation of the control member is controlledbased on the temperature in the primary exhaust emission control unit orthe secondary exhaust emission control unit. For example, if thecatalytic temperature in the primary exhaust emission control unitbecomes lower, the flow rate of the exhaust gas flowing through thefirst passage is increased by reducing the flow rate of the exhaust gasflowing through the second passage such that the thermal energy of theexhaust gas is used for increasing the catalytic temperature in theprimary exhaust emission control unit. Meanwhile if the catalytictemperature in the primary exhaust emission control unit becomes higher,the flow rate of the exhaust gas flowing through the second passage isincreased to reduce the flow rate of the exhaust gas flowing through thefirst passage so as to decrease the catalytic temperature. The thermalenergy of the exhaust gas is not used for increasing the catalytictemperature but collected by the first exhaust heat collecting unit.Accordingly the rise in the catalytic temperature owing to the exhaustgas at high temperature may be avoided, preventing thermal deteriorationin the catalyst. Furthermore, the thermal energy of the exhaust gas maybe efficiently used as the electric energy. The operation of the controlmember is controlled in accordance with the catalytic temperature in thesecondary exhaust emission control unit. The exhaust gas can be purifiedin the secondary exhaust emission control unit when the catalytictemperature therein reaches the activated temperature. Accordingly theflow rate of the exhaust gas flowing through the second passage isincreased to reduce the flow rate of the exhaust gas flowing through thefirst passage such that the thermal energy of the exhaust gas can becollected by the first exhaust heat collecting unit.

The exhaust system may include a heat exchange member in the secondpassage for transferring heat of the exhaust gas to the first exhaustheat collecting unit, and the heat exchange member is provided with acatalyst for purifying the exhaust gas.

The exhaust system may include the heat exchange member that transfersthe thermal energy of the exhaust gas to the high temperature side ofthe first exhaust heat collecting unit. The heat exchange member isprovided with the catalyst for purifying the exhaust gas flowing throughthe second passage. In the exhaust system, the exhaust gas is purifiedin the second passage, and the heat exchange member serves to absorb thereaction heat in the catalyst as well as the thermal energy of theexhaust gas. The resultant amount of the thermal energy collected by thefirst exhaust heat collecting unit is increased.

In the exhaust system, the first passage and the second passage may becombined into a single structure. The first passage is formed in acenter of the structure, and the second passage is formed on an outerperiphery of the first passage.

In the exhaust system, the second passage is formed on the outer side ofthe first passage formed in the center of the structure, and the firstexhaust heat collecting unit is provided on the outer side of the secondpassage. The first exhaust heat collecting unit is structured to convertthe thermal energy of the exhaust gas flowing through the second passageinto electric energy. In the exhaust system, the high temperatureexhaust gas (exhibiting higher thermal energy) flowing through the firstpassage formed in the center of the structure makes it possible torapidly increase the catalytic temperature.

In the exhaust system, the aforementioned structure may be placed in thevicinity of an exhaust manifold in an internal combustion engine.

The thermal energy of the exhaust gas discharged from the exhaustmanifold in the exhaust system is unlikely to be reduced in the vicinityof the exhaust manifold (downstream of the exhaust manifold throughwhich the high temperature exhaust gas is discharged). It is preferable,therefore, to provide the structure in the vicinity of the exhaustmanifold as close as possible. Alternatively, the structure may beprovided within the exhaust manifold if such arrangement is available.

In the exhaust system, the control member may serves to change each flowrate of the exhaust gas flowing into the first passage and the secondpassage from the internal combustion engine. In the aforementionedsystem, the flow rate of the exhaust gas flowing through the secondpassage can be adjusted.

The exhaust system is provided with a second exhaust heat collectingunit including a thermo electric element downstream of the secondaryexhaust emission control unit (thermoelectric converting member).

In the exhaust system, the second exhaust heat collecting unit isprovided for improving the efficiency of collecting the thermal energyof the exhaust gas as the electric energy. This makes it possible tocollect the thermal energy of the exhaust gas that has not beencollected in the first exhaust heat collecting unit, as the electricenergy, resulting in improved fuel efficiency. This may also allow theexhaust gas temperature to be further decreased, increasing engineoutputs.

In the exhaust system, the catalyst may be carried on the heat exchangemember.

The aforementioned exhaust system is provided with a heat exchange finof the heat exchange member. The catalyst is carried on the heatexchange fin for purifying the exhaust gas flowing through the secondpassage. The exhaust system purifies the exhaust gas in the secondpassage, and allows the heat exchange fin to absorb the reaction heat inthe catalyst in addition to the thermal energy of the exhaust gas so asto increase the amount of the thermal energy collected by the firstexhaust heat collecting unit.

In the exhaust system, the control member may be formed into a valvethat is operated to close and open the first passage and/or the secondpassage at a predetermined degree.

In the aforementioned exhaust system, each flow rate of the exhaust gasflowing through the first passage and the second passage can be finelyadjusted with the valve that is opened/closed at the predetermineddegree.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic overall view of an exhaust system according to afirst embodiment of the invention;

FIG. 2 is an exploded perspective view of a part of an upstream catalystshown in FIG. 1;

FIG. 3 is a side view of the upstream catalyst shown in FIG. 1;

FIG. 4 is a sectional view taken along IV-IV of the side view shown inFIG. 3;

FIG. 5 is an enlarged view of a portion around a discharge opening of aby-pass passage shown in FIG. 4;

FIG. 6 is a sectional view taken along VI-VI of the side view shown inFIG. 3;

FIG. 7 is a schematic overall view of an exhaust system according to asecond embodiment of the invention;

FIG. 8 is perspective view of an exhaust heat power generation apparatusshown in FIG. 7;

FIG. 9 is a front view of the exhaust heat power generation apparatus;

FIG. 10 is a sectional view taken along X-X of the front view shown inFIG. 9; and

FIG. 11 is a sectional view taken along XI-XI of the front view shown inFIG. 9.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of an exhaust system according to the invention will bedescribed referring to the drawings.

In this embodiment, an exhaust system is mounted on a vehicle forpurifying the exhaust gas from the engine while converting the thermalenergy of the exhaust gas into electric energy. The exhaust systemaccording to the embodiment is provided with two exhaust emissioncatalysts, one is placed in the vicinity of the exhaust manifold(hereinafter referred to as an upstream catalyst), and the other isplaced downstream thereof (hereinafter referred to as a downstreamcatalyst). The upstream catalyst in the vicinity of the exhaust manifoldis provided with an exhaust heat power generation unit. In thisembodiment, no additional exhaust heat power generation apparatus otherthan the aforementioned exhaust heat power generation unit is provided.In the other embodiment to be described later, an additional, exhaustheat power generation apparatus is provided.

A structure of the exhaust system of this embodiment will be describedreferring to FIG. 1 as a schematic overall view of the exhaust system.

An exhaust system 1 is mounted on a vehicle M, constituting an exhaustsystem downstream of an exhaust manifold EM of a four-cylinder engine(not shown). The exhaust system 1 mainly includes an upstream catalyst 2with an exhaust emission purifying function, e.g. a primary exhaustemission control unit, and a thermoelectric converting function, e.g. afirst exhaust heat collecting unit, a downstream catalyst 3 with anexhaust emission purifying function, e.g. a secondary exhaust emissioncontrol unit, at the downstream side, a sub muffler 4, and a mainmuffler 5. The exhaust gas discharged from each of the cylinders of theengine is joined at the exhaust manifold EM. The upstream catalyst 2provided in the vicinity of the exhaust manifold EM is connected with anexhaust pipe 6 a via a ball joint mechanism (not shown). The downstreamcatalyst 3 is provided at a downstream end portion of the exhaust pipe 6a. An exhaust pipe 6 b is connected with the downstream catalyst 3. Thesub muffler 4 is provided at a downstream end of the exhaust pipe 6 b.An exhaust pipe 6 c is connected with the sub muffler 4. The mainmuffler 5 is provided at a downstream end of the exhaust pipe 6 c via aball joint mechanism (not shown). A tail pipe (not shown) is provideddownstream of the main muffler 5.

The upstream catalyst 2 has an exhaust passage, as a first passage, thatpierces through the center thereof, and by-pass passages, as a secondpassage, at both sides of the exhaust passage. The exhaust passage ofthe upstream catalyst 2 is provided with the three-way catalyst forremoving the carbon monoxide, hydrocarbon, and the nitrogen oxides. Theupstream catalyst 2 admits the flow of the exhaust gas at a hightemperature (high thermal energy) from the exhaust manifold EM. In acold state upon start-up of the engine, the upstream catalyst 2 servesto purify the exhaust gas until the temperature of the three-waycatalyst of the downstream catalyst 3 reaches the activated temperatureto start purifying the exhaust gas. The upstream catalyst 2 is providedwith exhaust heat power generation units at both sides of the exhaustpassage, each of which converts the thermal energy of the exhaust gaspassing through the by-pass passage into electric energy so as to becharged in a battery (not shown) via a DC/DC converter (not shown) andthe like. When the upstream catalyst 2 is no longer required to purifythe exhaust gas with the three-way catalyst in the exhaust passage, thatis, the three-way catalyst provided in the downstream catalyst 3 isheated to reach the activated temperature so as to purify the exhaustgas, the upstream catalyst 2 starts conducting exhaust heat powergeneration. The structure of the upstream catalyst 2 will be describedin detail later.

The downstream catalyst 3 is filled with the three-way catalyst forremoving the carbon monoxide, hydrocarbon, and nitrogen oxides. As thedownstream catalyst 3 admits the exhaust gas at a temperature lower thanthat of the exhaust gas (lower thermal energy) passing through theupstream catalyst 2, it starts purifying the exhaust gas at a hightemperature in a high load of the engine, more specifically, at a timewhen the temperature of the three-way catalyst reaches the activatedtemperature to start purifying the exhaust gas.

The three-way catalyst provided in the upstream catalyst 2 is equivalentto that provided in the downstream catalyst 3. The three-way catalyst inthe upstream catalyst 2 is likely to be thermally degraded under thehigh heat of the exhaust gas. As a result, more three-way catalyst isprovided in the upstream catalyst 2 than in the downstream catalyst 3.The three-way catalyst is formed as a pellet of various metals or metaloxides and activated at the activated temperature ranging between 350°and 800° C., for example. The three-way catalyst exhibits the catalyticfunction in the aforementioned activated temperature range. Thetemperature of the three-way catalyst in each of the upstream catalyst 2and the downstream catalyst 3 is detected by a thermoelectric couple.Temperature signals US, DS each representing the detected temperature ofthe three-way catalyst in the upstream catalyst 2 and the downstreamcatalyst 3 are transmitted to an engine ECU (Electronic Control Unit) 7.

The sub muffler 4 of a small type is a noise eliminator that assists anoise eliminating function of the main muffler 5. The sub muffler 4 isprovided upstream of the main muffler 5 for reducing the acoustic energyof the exhaust gas. The sub muffler 4 does not have to be provided asthe exhaust noise in the exhaust system becomes relatively smallerresulting from each function of the upstream and downstream catalyst 2and 3, respectively compared with generally employed exhaust system.

The main muffler 5 is a main noise eliminator that is larger than thesub muffler 4 and has a greater noise eliminating function than that ofthe sub muffler 4. The main muffler 5 is provided downstream of the submuffler 4 for further reducing the acoustic energy which has beenreduced by the sub muffler 4 to a certain extent.

The engine ECU 7 includes a CPU (Central Processing Unit), a ROM (ReadOnly Memory), a RAM (Random Access Memory) and the like. The engine ECU7 sets various control values based on the detected values sent fromvarious sensors for controlling the engine and elements related thereto.The engine ECU 7 further controls the flow rate of the exhaust gasflowing through the exhaust passage and the by-pass passages in theupstream catalyst 2 of the exhaust system 1.

The structure of the upstream catalyst 2 will be described referring toFIGS. 2 to 6. FIG. 2 is an exploded perspective view of a part of theupstream catalyst. FIG. 3 is a side view of the upstream catalyst shownin FIG. 1. FIG. 4 is a sectional view taken along line IV-IV of the sideview shown in FIG. 3. FIG. 5 is an enlarged view of a portion around adischarge opening of a by-pass passage shown in FIG. 4. FIG. 6 is asectional view taken along line VI-VI of the side view shown in FIG. 3.

An inlet 20 of the upstream catalyst 2 at its upstream end is tightenedwith a discharge port EMa at the downstream end of the exhaust manifoldEM using a bolt (not shown). The inlet 20 has the same diameter as thatof the discharge port EMa, and its downstream end connected with a taperpipe 21. The taper pipe 21 has a diameter reduced as it goes downstream,and has a downstream end connected with a main body 22 of the upstreamcatalyst 2. The main body 22 of the upstream catalyst 2 is connectedwith a taper pipe 23 at its downstream end. The taper pipe 23 has adiameter reduced as it goes downstream. The taper pipe 23 is tightenedwith a ball joint mechanism at its downstream end with a bolt (notshown).

The main body 22 has a tubular shape extending from the upstream to thedownstream, which includes one exhaust passage 25 in the center thereofand two by-pass passages 26 at both sides of the exhaust passage 25 (seeFIGS. 4 and 6). An outer periphery of the main body 22 is provided withfour walls, that is, two side walls 22 a and two outer walls 22 b,respectively (see FIG. 6). Inner walls 22 c in parallel with the outerwall 22 b are provided at a predetermined interval within the main body22 (see FIG. 6). The exhaust passage 25 is defined by the side walls 22a and the inner walls 22 c, having a substantially rectangular shape asa sectional view and extending between the taper pipes 21 and 23. Eachof the by-pass passages 26 includes the side walls 22 a, outer wall 22b, the inner wall 22 c and two heat exchange members 29, having asubstantially rectangular shape as a sectional view and placed at theouter side of the exhaust passage 25.

The outer wall 22 b is provided with two heat exchange members 29 alongits longitudinal direction. Each of two openings 22 d is formed at aposition where the heat exchange member 29 is provided (see FIG. 2). Theopening 22 d has a substantially rectangular shape, into which a heatexchange fin 29 b of the heat exchange member 29 is inserted (FIGS. 2and 6). The outer wall 22 b has flanges 22 e at both outer sides of eachof the openings 22 d (see FIGS. 2 and 6). The outer wall 22 b has aplurality of bolt holes 22 f such that the outer periphery of theopening 22 d is tightened with the heat exchange member 29 and a coolingunit 31 with bolts 35 (see FIG. 2). Each of the bolt holes 22 f isprovided with a female thread.

The inner wall 22 c has an opening 22 g at its most upstream side, and adischarge opening 22 i at its most downstream side such that the exhaustpassage 25 is communicated with the by-pass passages 26 (see FIG. 4).The by-pass passage 26 admits the exhaust gas from the most upstreamportion of the exhaust passage 25 and discharges the exhaust gas intothe most downstream portion of the exhaust passage 25. An inner side ofthe inner wall 22 c has a door portion 22 j for opening/closing thedischarge opening 22 i as shown in FIGS. 4 and 5. The door portion 22 jhas a size sufficient to cover the discharge opening 22 i completelywhen it is closed, and is rotatably fixed at its end at the upstreamside with respect to a shaft 22 k. The door portion 22 j is opened underpressure of the exhaust gas flowing through the by-pass passage 26, andclosed under the pressure of the exhaust gas flowing through the exhaustpassage 25. The opening of the door portion 22 j, thus, is determineddepending on the gas pressure (flow rate of the gas) both in the by-passpassage 26 and the exhaust passage 25, respectively.

The pellet-like three-way catalyst is filled in the portion between theopening 22 g and the discharge opening 22 i each formed in the exhaustpassage 25, which constitutes a three-way catalyst 27 for purifying theexhaust gas, as the primary exhaust emission control unit. The exhaustpassage 25 includes a valve 22 m between the three-way catalyst 27 andthe exhaust opening 22 i as shown in FIG. 4. The valve 22 m has a size(corresponding to the sectional area of the exhaust passage 25)sufficient to cover the exhaust passage 25 completely. Then center ofthe valve 22 m is rotatably fixed with respect to the shaft 22 n that isrotated by an actuator (not shown). The actuator is driven in accordancewith a rotating driving signal RS from the engine ECU 7 as shown in FIG.1 so as to rotate the shaft 22 n (or the valve 22 m). The opening of thevalve 22 m is controlled by the engine ECU 7. When the valve 22 m isoperated to be at a right angle to the inner wall 22 c, the exhaustpassage 25 is completely closed. When the valve 23 m is operated to bein parallel with the inner wall 22 c (in full communication with theexhaust passage 25), the exhaust passage 25 is completely opened. Thevalve 22 m, shaft 22 n, the actuator and the door portion 22 j, and theshaft 22 k constitute a control member including a passageopening/closing structure.

The engine ECU 7 receives a temperature signal US from the upstreamcatalyst 2 and a temperature signal DS from the downstream catalyst 3,and sends a driving signal RS to an actuator for rotating the valve 22 m(see FIG. 1). The driving signal RS is sent by the engine ECU when thecatalytic temperature of the upstream catalyst 2 is lower than a lowerlimit at the upstream side (the lower limit of the activated temperatureof the three-way catalyst: 350° C., for example), and when the catalytictemperature of the upstream catalyst 2 is equal to or higher than thelower limit at the upstream side and the catalytic temperature of thedownstream catalyst 3 is lower than the lower limit at the downstreamside (the lower limit of the activated temperature of the three-waycatalyst: 350° C., for example) so as to open the valve 22 m fully. Inthis case, the valve 22 m is opened to open the exhaust passage 25 foradmitting the exhaust gas. The pressure of the exhaust gas serves tofully close the door portion 22 j so as to close the by-pass passages26. The flow of the exhaust gas into the by-pass passages 26 is blocked.When the catalytic temperature of the downstream catalyst 3 is equal toor higher than the lower limit at the downstream side or higher, thedriving signal RS is sent by the engine ECU 27. In this case. The valve22 m is fully closed to close the exhaust passage 25. Accordingly, theexhaust gas does not flow into the exhaust passage 25. The door portion22 j is fully opened under the pressure of the exhaust gas flowingthrough the by-pass passages 26 so as to be opened. The exhaust gas,thus, is allowed to flow through the by-pass passages 26. The drivingsignal RS for fully closing the valve 22 m may be sent from the engineECU 7 when the catalytic temperature of the upstream catalyst 2 becomeshigher than the upper limit at the upstream side (upper limit of theactivated temperature of the three-way catalyst: 800° C., for example).

Each of four exhaust heat power generation units 28 is fit in each ofthe openings 22 d formed in the main body 22 of the upstream catalyst 2.The exhaust heat power generation unit 28 is structured based on athermoelectric converting module 30. Each of the elements forconstituting the unit 28 is structured based on the size of thethermoelectric converting module 30. The exhaust heat power generationunit 28 applies appropriate pressures (17 kg/cm²) to the thermoelectricpower generation unit 28 from the low temperature side and the hightemperature side, thus improving the thermoelectric conversionefficiency. The exhaust heat power generation unit 28 is provided with aheat exchange member 29, the thermoelectric converting module 30 as thethermoelectric converting element, and a cooling unit 31.

The heat exchange member 29 is mainly formed of a base 29 a and a heatexchange fin 29 b as shown in FIG. 6. The base 29 a has a thickplate-like shape, and one flat surface which is in tight contact with ahigh temperature end surface of the thermoelectric converting module 30.The outer periphery of the base 29 a has bolt holes 29 c with which eachbolt 35 is fit for tightening the heat exchange member 29 with the mainbody 22 in engagement with the outer wall 22 b (flange portion 22 e) asshown in FIG. 2. The other surface of the base 29 a has the heatexchange fin 29 b attached thereto. The height of each fin of the heatexchange fin 29 b is set such that it is not in contact with the innerwall 22 c of the main body 22 to which the heat exchange member 29 isattached as shown in FIG. 4. The surface area of the heat exchange fin29 b of the heat exchange member 29 is enlarged to increase the area ofthe portion in contact with the exhaust gas such that more thermalenergy of the exhaust gas is absorbed. Each of the heat exchange members29 is fit with the corresponding opening 22 d formed in the main body 22so as to be tightened together with each of the cooling units 31 usingthe respective bolts 35 (see FIG. 2). This makes it possible to formeach of the by-pass passages 26 (see FIG. 4).

The pellet-like three-way catalyst that is identical to the three-waycatalyst 27 may be burned into the surface of each fin of the heatexchange fin 29 b so as to be carried on the heat exchange member 29.The aforementioned structure makes it possible to purify the exhaust gasflowing through the by-pass passage 26 while converting the thermalenergy into electric energy. In this case, the heat exchange fin 29 b iscapable of absorbing the thermal energy resulting from the reaction heatcaused by the three-way catalyst as well as the thermal energy of theexhaust gas. This makes it possible to improve the thermoelectricconversion efficiency of the exhaust heat power generation unit 28.

The thermoelectric converting module 30 includes a plurality ofthermoelectric elements (not shown), for example, semiconductors of ptype and n type formed of Bi₂Te₃ which are arranged in serieselectrically and in parallel thermally. The thermoelectric convertingmodule 30 has a substantially square shape with small area, having ahigh temperature end surface and a low temperature end surfacehorizontally arranged in parallel with each other. The thermoelectricconverting module 30 serves to convert the thermal energy into theelectric energy using the temperature difference between both endsurfaces based on Seebeck effect such that electric energy is outputfrom two electrodes (not shown).

The cooling unit 31 serves to apply appropriate pressure to a lowtemperature end surface of the thermoelectric converting module 30 so asto be fixed and cooled with water. The cooling unit 31 is provided witha lid 32, a main body 33 and cooling water pipes 34.

The lid 32 for the main body 33 has a thick plate portion 32 a with thesame dimension as that of the main body 33 (see FIGS. 2 and 3). Theplate portion 32 a has mount portions 32 b at both sides on which thecooling water pipes 34 are attached (see FIGS. 2 and 3). Each of themount portions 32 b has a mounting hole 32 c with which the coolingwater pipe 34 is fit and a cooling water hole 32 d connected to thelower side of the mounting hole 32 c (see FIG. 4). Each of the coolingwater holes 32 d pierces to the bottom face of the lid 32 so as to beconnected with a cooling portion 33 a. Bolt holes (not shown) are formedin each corner of the plate portion 32 a, through which the main body 33and the heat exchange member 29 are attached to the main body 22 (seeFIG. 2) with the respective bolts 35.

The main body 33 is formed into a thick box like shape to be closed withthe lid 32, which has a dimension slightly longer than that of thethermoelectric converting module 30. A recess portion of the main body33 constitutes the cooling portion 33 a into which the cooling waterflows (see FIG. 6). The cooling portion 33 a is provided with a coolingfin 33 b for cooling the cooling water. Each fin of the cooling fin 33 bhas the same height so as to be in contact with the bottom of the lid 32when being set onto the main body 33. The bottom surface of the mainbody 33 is flat so as to be in tight contact with the low temperatureend surface of the thermoelectric converting module 30. Bolt holes (notshown) are formed in each corner of the main body 33 through which thelid 32 and the heat exchange member 29 are attached to the main body 22,and then tightened with bolts 35 (see FIG. 2).

The lid 32 is set to be fixed on the main body 33 by tightening withfour bolts 35 (see FIG. 2), and two cooling water pipes 34 are attachedto the lid 32 by welding to form the cooling unit 31. The cooling unit31 is further fixed to the body 22 via the thermoelectric convertingmember 29 with four bolts 35. The use of those bolts 35 makes itpossible to interpose the thermoelectric converting module 30 betweenthe cooling unit 31 and the heat exchange member 29 under theappropriate surface pressure.

The upstream catalyst 2 is provided with two cooling units 31 arrangedin the longitudinal direction. The cooling water pipe 34 upstream of thecooling unit 31 at the upstream side and the cooling water pipe 34downstream of the cooling unit 31 at the downstream side are connectedto a radiator (not shown) via a radiator hose (not shown), and the othercooling water pipes 34 are connected with each other (see FIG. 2). Inthe cooling unit 31, the cooling water cooled by the radiator isadmitted into the cooling portion 33 a through the cooling water pipe 34and the cooling water hole 32 d. The cooling water then flows throughfins of the cooling fin 33 b so as to cool the thermoelectric convertingmodule 30, keeping the low temperature (see FIG. 4).

An operation of the exhaust system 1 will be described referring toFIGS. 1 to 6. Each operation of the exhaust system 1 will be describedin the case where:

-   (1) the catalytic temperature in the upstream catalyst 2 is lower    than the lower limit at the upstream side;-   (2) the catalytic temperature in the upstream catalyst 2 is equal to    or higher than the lower limit at the upstream side, and the    catalytic temperature in the downstream catalyst 3 is lower than the    lower limit at the downstream side; and-   (3) the catalytic temperature in the downstream catalyst 3 is equal    to or higher than the lower limit at the downstream side.    (1) The operation of the exhaust system 1 will be described in the    case where the catalytic temperature in the upstream catalyst 2 is    lower than the lower limit at the downstream side. Upon start-up of    the engine, the exhaust gas is discharged from each cylinder of the    engine. The exhaust gas is admitted by the upstream catalyst 2 via    the exhaust manifold EM. In the upstream catalyst 2, the exhaust gas    flows into the exhaust passage 25 passing through the three-way    catalyst 27.

As the temperature of the exhaust system as a whole is low at thestart-up of the engine, the catalytic temperature of the three-waycatalyst 27 is lower than the lower limit at the upstream side, that is,the catalytic temperature has not reached the activated temperature yet.The engine ECU 7 sends the driving signal RS for fully opening the valve22 m to the actuator based on the temperature signal US from theupstream catalyst 2. Then the actuator rotates the shaft 22 n to fullyopen the valve 22 m. When the valve 22 m is fully opened, the exhaustpassage 25 is opened to allow the exhaust gas to flow therethrough. Theresultant pressure of the exhaust gas brings the door portion 22 j intoa fully closed state to close the by-pass passages 26, interrupting theflow of the exhaust gas thereinto. Therefore, each of the exhaust heatpower generation unit 28 does not generate power using the exhaust heat.At this moment, the cooling water is not circulated in the cooling unit31 of the exhaust heat power generation unit 28.

After passing through the upstream catalyst 2, the exhaust gas isadmitted by the downstream catalyst 3. When the exhaust gas is admittedby the downstream catalyst 3 to pass through the three-way catalyst, thecatalytic temperature is lower than the lower limit at the downstreamside, that is, the catalytic temperature has not reached the activatedtemperature yet.

After passing through the downstream catalyst 3, the exhaust gas flowsthrough the sub muffler 4 and the main muffler 5 where the noise causedby the flow of the exhaust gas is eliminated. The exhaust gas having itsnoise eliminated is then discharged into atmosphere.

In this case, as the by-pass passages 26 are closed, the thermal energyof the exhaust gas can be used only for increasing the temperature ofthe three-way catalyst without being consumed by the exhaust eat powergeneration units 28. As a result, the catalytic temperature of thethree-way catalyst 27 may be rapidly increased as the rise in theexhaust gas temperature.

(2) The operation of the exhaust system 1 will be described in the casewhere the catalytic temperature in the upstream catalyst 2 is equal toor higher than the lower limit at the upstream side and the catalytictemperature in the downstream catalyst 3 is lower than the lower limitat the downstream side. After start-up of the engine, the temperature ofthe exhaust system as a whole is gradually increased. The exhaust gas isadmitted by the upstream catalyst 2 via the exhaust manifold EM. Theexhaust gas then flows into the exhaust passage 25 of the upstreamcatalyst 2 passing through the three-way catalyst 27. As the upstreamcatalyst 2 is placed in the vicinity of the exhaust manifold EM, theexhaust gas may flow into the exhaust passage 25 while being held at thehigh temperature. Accordingly the catalytic temperature of the three-waycatalyst 27 sharply rises, reaching the activated temperature within aconsiderably short period. Subsequent to the moment when the catalytictemperature reaches the activated temperature, the three-way catalyst 27starts purifying the exhaust gas, At this time, the catalytictemperature of the three-way catalyst 27 becomes equal to or higher thanthe lower limit at the upstream side.

After passing through the upstream catalyst 2, the exhaust gas has itsthermal energy consumed to heat the three-way catalyst 27. The exhaustgas at the decreased temperature is admitted by the downstream catalyst3. The exhaust gas is admitted to pass through the three-way catalyst inthe downstream catalyst 3. The catalytic temperature of the downstreamcatalyst 3 is gradually increased, but is lower than the lower limit atthe downstream side, that is, the catalytic temperature has not reachedthe activated temperature. The engine ECU 7 sends the driving signal RSfor fully opening the valve 22 m to the actuator based on thetemperature signal US from the upstream catalyst 2 and the temperaturesignal DR from the downstream catalyst 3. As the valve 22 m is in thefully opened state, the exhaust passage 25 is opened. As the doorportion 22 j is in the fully closed state, the by-pass passages 26 areclosed. In the aforementioned case, the engine ECU 7 may be structuredto send the driving signal RS to the actuator so as to graduallyreducing the opening of the valve 22 m in accordance with the catalytictemperature either of the upstream catalyst 2 or the downstream catalyst3. This may allow the exhaust gas to gradually increase the flow rate ofthe exhaust gas flowing into the by-pass passages 26. As a result, poweris generated by the exhaust heat power generation units 28, andexcessive rise in the catalytic temperature of the upstream catalyst 2may be prevented.

After passing through the downstream catalyst 3, the exhaust gas flowsinto the sub muffler 4 and the main muffler 5 where the noise caused bythe flow of the exhaust gas is eliminated. The exhaust gas having beenpurified and noise muffled is discharged into atmosphere.

In the aforementioned case, although the catalytic temperature in thedownstream catalyst 3 has not reached the activated temperature, theupstream catalyst 2 has purified the exhaust gas to a certain extent. Asthe by-pass passages 26 are closed, the thermal energy of the exhaustgas may be used only for heating the three-way catalyst in both theupstream catalyst 2 and the downstream catalyst 3 without being consumedby the exhaust heat power generation units 28. This makes it possible toincrease the catalytic temperature in the downstream catalyst 3 rapidlyas the rise in the exhaust gas temperature. If the downstream catalyst 3exhibits the purifying capability partially, it is capable of purifyingthe exhaust gas that has passed through the by-pass passages 26 each inpartly opened state.

(3) The operation of the exhaust system 1 will be described in the casewhere the catalytic temperature in the upstream catalyst 3 is equal toor higher than the lower limit at the downstream side. In the high loadof the engine (at high engine speed), the exhaust gas at the hightemperature is admitted by the upstream catalyst 2 and the downstreamcatalyst 3. Each of the catalytic temperature in the upstream and thedownstream catalysts 2 and 3 has reached the activated temperature.

The catalytic temperature in the downstream catalyst 3 becomes equal toor higher than the lower limit at the downstream side, that is, thecatalytic temperature has reached the activated temperature. The engineECU 7 sends the driving signal RS to the actuator for fully closing thevalve 22 m based on the temperature signal DS from the downstreamcatalyst 3. In response to receipt of the driving signal RS, theactuator rotates the shaft 22 n such that the valve 22 m is fullyclosed. When the valve 22 m is fully closed, the exhaust passage 25 isclosed. At this moment, as the flow of the exhaust gas into the exhaustpassage 25 is interrupted, the three-way catalyst 27 does not purify theexhaust gas.

When the exhaust gas flows into the by-pass passages 26, the doorportion 22 j is brought into the fully closed state under the pressureof the exhaust gas to open the by-pass passages 26 so as to allow theflow of the exhaust gas. In each of the exhaust heat power generationunits 28, the thermal energy of the exhaust gas flowing through theby-pass passage is absorbed in fins of the heat exchange fin 29 b of theheat exchange member 29. The high temperature is then transferred to thehigh temperature end surface of the thermoelectric converting module 30.In each of the exhaust heat power generation units 28, the cooling wateris circulated in the cooling unit 31 such that the low temperature istransferred to the low temperature end surface of the thermoelectricconverting module 30. Finally the exhaust heat power generation units 28serve to generate power in accordance with the difference in the hightemperature and the low temperature in the thermoelectric convertingmodule 30. The generated power is then charged in the battery.

In the case where the three-way catalyst is carried on the heat exchangefin 29 b, the exhaust gas flowing through the by-pass passages 26 ispurified by the three-way catalyst. The reaction heat caused by thethree-way catalyst is absorbed by the heat exchange fin 29 b.

After passing through the upstream catalyst 2, the exhaust gas isadmitted by the downstream catalyst 3. As the catalytic temperature inthe downstream catalyst 3 has reached the activated temperature, theexhaust gas can be purified thereby. The exhaust gas that has passedthrough the downstream catalyst 3 is admitted by the sub muffler 4 andthe main muffler 5 where the noise of the exhaust gas is eliminated. Theexhaust gas that has been purified and noise muffled is discharged intoatmosphere.

As the catalytic temperature in the downstream catalyst 3 has reachedthe activated temperature, the exhaust gas can be purified. The by-passpassages 26 are opened such that the thermal energy of the exhaust gasis collected through power generation performed by the exhaust heatpower generation units 28 without being consumed for increasing thecatalytic temperature of the three-way catalyst 27 in the upstreamcatalyst 2. As a result, the catalytic temperature of the three-waycatalyst 27 is not increased, preventing the catalytic temperature fromexceeding the upper limit of the activated temperature.

In the exhaust system 1, the upstream catalyst 2 is placed in thevicinity of the exhaust manifold EM, having the exhaust passage 25filled with the three-way catalyst in the center thereof. The exhaustgas exhibiting the highest thermal energy among the elementsconstituting the exhaust system 1 is admitted by the three-way catalyst27. This makes it possible to raise the catalytic temperature to theactivated temperature rapidly even if it is lower than the activatedtemperature. The exhaust system 1 includes by-pass passages 26 eachprovided at both sides of the exhaust passage 25. The aforementionedby-pass passages 26 provide the heat retention and thermal insulationfunctions, thus further enhancing the temperature increasing effect ofthe three-way catalyst 27.

The exhaust system 1 is provided with the exhaust heat power generationunits 28 for collecting the thermal energy of the exhaust gas flowingthrough the by-pass passages 26 in addition to the exhaust passage 25 inthe upstream catalyst 2. In the case where the exhaust gas is notrequired for heating the three-way catalyst 27, the exhaust gas isadmitted into the by-pass passages 26 so as to be used for collectingthe thermal energy of the exhaust gas. In the exhaust system 1, as theupstream catalyst 2 is placed in the vicinity of the exhaust manifoldEM, the electric energy can be efficiently collected from the exhaustgas exhibiting the highest thermal energy. As the exhaust system 1 isprovided with the downstream catalyst 3, the flow of the exhaust gasinto the exhaust passage 25 in the upstream catalyst 2 can beinterrupted when the catalytic temperature of the downstream catalyst 3reaches the activated temperature. This makes it possible to prevent thecatalytic temperature in the upstream catalyst 2 from increasing inexcess of the activated temperature, avoiding degradation of thethree-way catalyst. There is no needs of operating the engine in afuel-rich state for reducing the catalytic temperature in the upstreamcatalyst 2. This may prevent the fuel from being excessively consumedyet collecting the high thermal energy as electric energy, resulting inimproved fuel efficiency. As the three-way catalyst is unlikely to bedegraded, the amount of the three-way catalyst can be reduced, thusmaking each of the upstream and the downstream catalysts 2, 3 intocompact.

The exhaust system 1 is allowed to control the flow rate of the exhaustgas passing- through each of the exhaust passage 25 and the by-passpassages 26, respectively in accordance with the respective catalytictemperatures in the upstream and the downstream catalysts 2, 3. Thismakes it possible to allow both the three-way catalyst 27 and theexhaust heat power generation units 28 to be operated in the vicinity ofthe exhaust manifold EM, resulting in efficient use of the thermalenergy of the exhaust gas.

In the exhaust system 1 having the three-way catalyst carried on theheat exchange fin 29 b, the catalytic effect may be enhanced and theeffect for absorbing the thermal energy resulting from the reaction heatof the three-way catalyst may be improved. In this case, the exhaust gasthat has escaped into the by-pass passages 26 can be reliably purified.Accordingly, the exhaust system 1 is capable of performing its functionsufficiently even if the downstream catalyst 3 is not provided.

In the exhaust system 1, the noise caused by the flow of the exhaust gascan be reduced, thus reducing each size of the sub muffler 4 and themain muffler 5. The exhaust system 1 is capable of performing itsfunction sufficiently without the sub muffler 4. The exhaust system 1exhibits excellent efficiency for collecting the thermal energy of theexhaust gas, which may reduce the temperature and the volume of theexhaust heat. The resultant decrease in the flow rate of the gas mayallow the resistance against the exhaust gas pressure to be decreased,increasing the engine outputs.

A structure of an exhaust system 41 according to another embodiment willbe described referring to FIG. 7. FIG. 7 is a schematic view of theexhaust system according to this embodiment. The elements constitutingthe exhaust system 41 identical to those of the exhaust system 1 aredesignated with the same reference numerals in the previous embodiment,and each description thereof, thus, is omitted.

The structure of the exhaust system 41 is the same as that of theexhaust system 1 except that an exhaust heat power generation apparatus42, as a second exhaust heat collecting unit, is provided in place ofthe sub muffler 4 (see FIG. 1). As has been already described in theprevious embodiment, the noise produced by the exhaust system accordingto this embodiment is smaller than that produced by the generallyemployed exhaust system. In this embodiment, the exhaust system 41 isprovided with an exhaust heat power generating apparatus 42 thatfunctions as a sub muffler so as to improve the efficiency forcollecting the thermal energy of the exhaust gas as electric energy. Theexhaust system 41 is mainly formed of the upstream catalyst 2, thedownstream catalyst 3, the exhaust heat power generation apparatus 42,and the main muffler 5. The exhaust heat power generation apparatus 42is placed at the position where the sub muffler 4 of the exhaust system1 is provided. Exhaust pipes 6 b and 6 c are connected to the upstreamend and the downstream end of the exhaust heat power generationapparatus 42, respectively.

The exhaust heat power generation apparatus 42 converts the thermalenergy of the exhaust gas that has not been collected by the upstreamcatalyst 2 into electric energy. The resultant electric energy ischarged in the battery via the DC/DC converter.

The structure of the exhaust heat power generation apparatus 42 will bedescribed referring to FIGS. 8 to 11. FIG. 8 is a perspective view ofthe exhaust heat power generation apparatus. FIG. 9 is a front view ofthe exhaust heat power generation apparatus shown in FIG. 7. FIG. 10 isa sectional view taken along line X-X of the front view shown in FIG. 9.FIG. 11 is a sectional view taken along line XI-XI of the front viewshown in FIG. 9.

The exhaust heat power generation apparatus 42 has an exhaust pipe oftetrameric type (divided into four parts along the periphery). There arefour exhaust heat power generation units 43 arranged along the periphery(see FIG. 10). Each exhaust heat power generation units 43 in theexhaust heat power generation apparatus 42 serves to convert the thermalenergy of the exhaust gas into electric energy.

In the exhaust heat power generation apparatus 42, an exhaustintroduction pipe 44 connected to the exhaust pipe 6 b at upstream sideis provided at an end of the upstream side, and an exhaust dischargepipe 45 connected to an exhaust pipe at downstream side is provided atan end of the downstream side. There are four divided exhaust pipes 46which are connected with one another through welding between the exhaustintroduction pipe 44 and the exhaust discharge pipe 45. Each of thosedivided exhaust pipes 46 is arranged at every 90° at the center of theexhaust heat power generation apparatus 42, forming each frame of fourdivided exhaust passages 47, respectively (see FIG. 10).

The divided exhaust pipe 46 has a main portion of a thin plate-likeshape, and an isosceles trapezoidal shape as a side view (see FIG. 10).Each angle defined by the long side and two sides each connectingbetween the respective ends of the long side and the short side is 45°.An outer plate corresponding to the long side of the divided exhaustpipe 46 is provided with openings. Each of the openings has asubstantially square shape, to which a heat exchange fin 48 b of theheat exchange member 48 is inserted. The outer plate has bolt holesformed therein through which the heat exchange member 48 is tightenedwith a bolt along the outer periphery of the opening.

Side plates of each of the divided exhaust pipes 46 are welded to therespective side plates of adjacent divided exhaust pipes each arrangedat 90°. Then four divided exhaust pipes 46 are connected along theperiphery and formed into a substantially square shape as the side view(see FIG. 10). Each of the divided exhaust pipes 46 is provided withheat exchange member 48. A divided exhaust passage 47 is formed byclosing the opening. Both ends of each of inner plates of four dividedexhaust pipes 46 are connected to a split member 46 b at an upstreamside through welding, and a joint member 46 b at a downstream sidethrough welding (see FIG. 11). The split member 46 a is formed like aquadrangular pyramid shape having its width reduced as it goes upstreamsuch that the flow of the exhaust gas from the exhaust introduction pipe6 b is split into four divided exhaust passages 47. The joint member 46b is formed like a quadrangular pyramid shape having its width reducedas it goes downstream such that the exhaust gas each flowing throughfour divided exhaust passages 47 is joined together.

The exhaust heat power generation unit 43 is mainly formed of athermoelectric converting module 49. Various elements that constitutethe unit 43 are formed based on the size of the thermoelectricconverting module 49. In the exhaust heat power generation unit 43,appropriate pressure (for example, 17 kgcm²) is applied to thethermoelectric converting module 49 from the low temperature side andthe high temperature side. The exhaust heat power generation unit 43 isflexibly compressed by the spring system so as to improve thethermoelectric conversion efficiency of the thermoelectric convertingmodule 49. Then each of the exhaust heat power generation unit 43 is fitin the opening of the divided exhaust pipe 46, respectively. The exhaustheat power generation unit 43 is provided with the heat exchange member48, the thermoelectric converting module 49, a cooling unit 50, and aspring clamp 54.

The heat exchange member 48 is mainly formed of a base 48 a and a heatexchange fin 48 b. The base 48 a has a dimension longer than that of thethermoelectric converting module 49 both in the width and longitudinaldirections. The upper surface of the base 48 a has a flat face so as tobe brought into tight contact with the high temperature end surface ofthe thermoelectric converting module 49. The outer peripheral portion ofthe base 48 a has a plurality of holes formed therein, through which thedivided exhaust pipe 46 is fixed and tightened with bolts. The heatexchange fin 48 b attached to the base 48 a has a height such that eachfin of the heat exchange fin 48 b becomes close to the side plates andthe inner plates of the divided exhaust pipe 46 but not in contacttherewith when the heat exchange member 48 is attached to the dividedexhaust pipe 46. Therefore all the fins of the heat exchange fin 48 bform substantially isosceles trapezoidal shape as shown in FIG. 10. Theheat exchange member 48 is fit into the opening of the divided exhaustpipe 46 and tightened with bolts 58 so as to form the divided exhaustpassage 47 (see FIGS. 10, 11). The height of each fin of the heatexchange fin 48 b may be set so as to be in contact with the side plateand inner plate of the divided exhaust pipe 46. In this case, however,each of the fins or the divided exhaust pipe 46 has to be formed to besufficiently deformed, and have the exhaust gas absorbing action.

The thermoelectric converting module 49 is identical to thethermoelectric converting module 30 as described in the previousembodiment.

The cooling unit 50 serves to apply appropriate pressure to a lowtemperature end surface of the thermoelectric converting module 49 so asto be fixed and cooled with water. The cooling unit 50 is provided witha lid 51, a main body 52 and cooling water pipes 53.

The lid 51 for the main body 52 has a plate portion. A circular bottomedhole is formed in the center of the plate portion so as to receive acompression member 57 to be fit therein. Support portions 9 are providedat both sides of the hole, each of which serves to support leaf springs56 from both sides and to place cooling water pipes 53. Fitting holesare formed in the outer side of the respective support portions suchthat the respective cooling water pipes 53 are fit therein. Coolingwater holes are further formed in the support portions, eachcommunicated with lower side portion of the fitting holes, respectively.The cooling water holes pierce through the bottom of the lid 51 so as tobe communicated with a cooling portion 52 of the main body 52 (see FIG.11). Holes are formed at each corner of the plate portion through whichthe main body 52 is tightened with bolts.

The main body 52 has a box-like shape with large thickness. A recessportion of the main body 52 constitutes a cooling portion 52 into whichthe cooling water flows (FIG. 11). The cooling portion 52 a is providedwith a cooling fin 52 b for cooling the cooling water. Each fin of thecooling fin 52 b has the same height so as to be in contact with thebottom of the lid 51 when being set onto the main body 52. The bottomsurface of the main body 52 is flat so as to be in tight contact withthe low temperature end surface of the thermoelectric converting module49. Bolt holes are formed in each corner of the main body 52 throughwhich the lid 51 is tightened with the bolt.

The lid 51 is set to be fixed on the main body 52 by tightening fourbolts (not shown), and two cooling water pipes 53 are attached to thelid 51 by welding to form the cooling unit 50. The exhaust heat powergeneration apparatus 42 has four cooling units 50 arranged in theperipheral direction (see FIG. 10). The two cooling water pipes 53respectively attached to the adjacent cooling units 50 are connected toa radiator (not shown) via a radiator hose (not shown). The othercooling water pipes 53 are connected via a connecting pipe 53 betweenthe adjacent cooling units 50 (see FIG. 8). In each of the cooling units50, the cooling water that has been cooled by the radiator is admittedinto the cooling portion 52 a through the cooling water pipe 53 and thecooling water hole, and further allowed to pass through fins of thecooling fin 52 b. The thermoelectric converting module 49 is cooled, andthe low temperature, thus, can be kept.

The spring clamp 54 serves to apply a predetermined pressure to thethermoelectric converting module 49 from the outside of the cooling unit50 so as to be fixed between the cooling unit 50 and the heat exchangemember 48. At this time, the spring clamp 54 flexibly compresses theexhaust heat power generation unit 43 as a whole with the elastic forcederived from a plurality of leaf springs. Four spring clamps 54 arefastened to the exhaust heat power generation apparatus 42 along itsperiphery so as to be tightened. The spring clamp 54, therefore,includes a clamp 55, a plurality of leaf springs 56, and the compressionmember 57.

The clamp 55 includes a storage portion 55 a, connecting portions 55 b,fastening portions 55 c (see FIG. 8). The storage portion 55 a,connecting portions 55 b and the fastening portions 55 c are all formedfrom a single plate. The storage portion 55 a has a recess portion whenviewed in front. An opening 55 d that has the same shape but slightlysmaller size than those of the leaf springs 56 is formed in the centerof the storage portion 55 a (FIG. 8). The outer periphery of the opening55 d serves to compress the leaf spring 56. The connecting portions 55 bserve to connect the storage portion 55 a to the fastening portions 55 cat both ends. The fastening portion 55 c is formed by bending both endsof the plate that constitutes the clamp 55 at right angles with respectto the connecting portions 55 b so as to be in contact with the adjacentbottom surface of the clamp 55. Three bolt holes through which boltspierce are formed in each of the fastening portions 55 c. When fourclamps 15 are connected and fastened, they form a substantially circularshape as a sectional view, covering the outermost portion of the exhaustheat power generation apparatus 42 (see FIG. 10).

The leaf spring 56 has a substantially ellipsoidal shape as a plan view.As the leaf spring 56 has a small spring constant, the spring clamp 54generates the elastic force by stacking a plurality of the leaf springs56.

The compression member 57 has a semi-spherical shape so as to be inpoint contact with the leaf spring 56. The circular bottom surface ofthe compression member 57 has a size sufficient to be fit with the holeformed in the lid 51.

In the spring clamp 54, the compression member 57 is fit with the holeof the lid 51 of the cooling unit 50. A plurality of leaf springs 56 arestacked on the compression member 57. The clamp 55 is placed on the leafsprings 56 so as to be covered with the storage portion 55a. The leafsprings 56 are supported by the support portions of the lid 51 at bothsides. The top surface of the stacked leaf springs 56 is higher than thesupport portions. The fastening portions 55 c of the clamp 55 arealigned with respect to the fastening portions 55 c of the clamps 55 atboth sides such that the fastening portions 55 c of the adjacent clamps55 are fastened with bolts 59 and nuts 60 as shown in FIG. 10. Theexhaust heat power generation apparatus 42 is fastened with four springclamps 54 that are fastened along the periphery and functioning like abelt.

An operation of the exhaust system 41 will be described referring toFIGS. 7 to 11. The exhaust system 41 of this embodiment is the same asthe exhaust system 1 in the previous embodiment except that the exhaustsystem 41 generates power without noise muffling by the sub muffler 4.The operation of the exhaust system 41 that is different from that ofthe exhaust system 1 will be described hereinafter.

In the exhaust heat power generation apparatus 42, the exhaust gas flowsfrom the exhaust introduction pipe 44, and the cooling water flows fromthe cooling water pipes 53 connected to the radiator. The introducedexhaust gas is split by the split member 46 a into each of the fourdivided exhaust passages 47. The introduced exhaust gas has beenpartially used for heating the three-way catalyst in the upstreamcatalyst 2 or consumed to be converted into electric energy. Therefore,the thermal energy of the introduced exhaust gas is lower than that ofthe exhaust gas to be introduced into the upstream catalyst 2.

In each of the divided exhaust passages 27, the exhaust gas passesthrough fms of the heat exchange fin 48 b of the heat exchange member 48and then flows downstream. The thermal energy of the exhaust gas isabsorbed by the heat exchange fin 48 b as it passes therethrough. Theheat exchange member 48 transfers the absorbed energy to the hightemperature end surface of the thermoelectric converting module 49.

The cooling water passes through fins of the cooling fin 52 b in each ofthe cooling portions 52 a of the cooling units 50, and flows downstream.The cooling unit 50 transfers the cooling water at the low temperatureto the low temperature end surface of the thermoelectric convertingmodule 49.

Each of the thermoelectric converting modules 49 generates power inaccordance with the temperature difference between the high temperaturetransferred to the high temperature end surface and the low temperaturetransferred to the low temperature end surface. The resultant electricenergy is charged in the battery. As the high temperature and the lowtemperature can be sufficiently held and the temperature differencebecomes large, the generated power becomes substantially high.

Subsequent to collection of the thermal energy of the exhaust gas as theelectric energy in the exhaust heat power generation apparatus 42, theexhaust gas is introduced into the main muffler 5. As the thermal energyof the exhaust gas has been already collected, the temperature thereof,thus, is lowered.

The exhaust system 41 is capable of collecting the thermal energy of theexhaust gas as electric energy in addition to the effect obtained by theprevious embodiment, resulting in improved fuel efficiency. As theexhaust gas temperature is further reduced, the engine outputs can beimproved.

As has been described with respect to the embodiments, it is to beunderstood that the invention may be embodied in various forms withoutbeing limited thereto.

In this embodiment, two catalytic systems are provided. However, theinvention may be structured to include one catalytic system providedwith the exhaust heat power generation unit. In the above case, thethree-way catalyst is carried on the heat exchange fin in order topurify the exhaust gas that passes through the by-pass passages. In eachof the aforementioned two catalytic systems, the same three-way catalystis used for purifying the exhaust gas. However, different catalysts maybe used in the respective catalytic systems in consideration withcharacteristics of the engine for purifying the exhaust gas. Forexample, one of the catalytic systems may use the oxidation catalyst,and the other may use the reduction catalyst.

In this embodiment, the catalytic system with the exhaust heat powergeneration unit is placed in the vicinity of the exhaust manifold.However, the catalytic system may be placed on an arbitrary position inthe exhaust system. Alternatively the catalytic system may be placedwithin the exhaust manifold at the most downstream portion.

In the exhaust system according to this embodiment, two catalyticsystems are provided at the upstream and the downstream sides,respectively. Then the valve opening is controlled based on thetemperature in the catalysts in the respective catalytic systems.However, only one catalytic system with the exhaust heat powergeneration unit may be provided so as to control the valve opening suchthat the catalytic temperature becomes within the activated temperaturerange.

In this embodiment, the valve provided in the exhaust passage iscontrolled to be fully closed/opened. However, the valve opening may becontrolled stepwise in accordance with the catalytic temperature. Thevalve to be driven by the actuator may be provided in the by-passpassage such that the valve opening can be controlled in accordance withthe catalytic temperature.

In this embodiment, not only the cooling unit of water-cooling type butalso air-cooling type may be employed.

The exhaust system of the invention may prevent degradation of thecatalyst and collect the thermal energy from the exhaust gasefficiently, resulting in improved fuel efficiency.

1. An exhaust system comprising: an exhaust passage that allows exhaustgas discharged from an internal combustion engine to pass therethrough;a primary exhaust emission control unit including a catalyst to purifythe exhaust gas; a first exhaust heat collecting unit including athermoelectric element that converts thermal energy of the exhaust gasinto electric energy, the exhaust passage being divided into a firstpassage provided with the primary exhaust emission control unit and asecond passage provided with the first exhaust heat collecting deviceincluding the thermoelectric element; and a control member that isoperated to change a flow of the exhaust gas between the first passageand the second passage; wherein an operation of the control member iscontrolled based on a temperature in the primary exhaust emissioncontrol unit; the control member is operated such that the exhaust gasflows through the second passage when the temperature in the primaryexhaust emission control unit exceeds a predetermined temperature; andthe predetermined temperature is determined based on an activationtemperature of the catalyst in the primary exhaust emission controlunit.
 2. The exhaust system according to claim 1, further comprising asecondary exhaust emission control unit provided on the exhaust passagewhere the first passage and the second passage are joined.
 3. Theexhaust system according to claim 2, wherein an operation of the controlmember is controlled based on a temperature in the secondary exhaustemission control unit.
 4. The exhaust system according to claim 3,wherein the control member is operated such that the exhaust gas flowsthrough the second passage when the temperature in the secondary exhaustemission control unit exceeds a predetermined temperature.
 5. Theexhaust system according to claim 4, wherein the predeterminedtemperature is determined based on an activation temperature of thecatalyst in the secondary exhaust emission control unit.
 6. The exhaustsystem according to claim 2, further comprising a second exhaust heatcollecting unit including a thermoelectric element downstream of thesecondary exhaust emission control unit.
 7. The exhaust system accordingto claim 1, wherein: the first passage and the second passage arecombined into a single structure; the first passage is provided in acenter of the structure; and the second passage is provided on an outerperiphery of the first passage.
 8. The exhaust system according to claim1, wherein: the second passage includes a heat exchange member thattransfers heat of the exhaust gas to the exhaust heat collecting device;and the exhaust heat collecting device is provided with a catalyst forpurifying the exhaust gas.
 9. The exhaust system according to claim 8,wherein the catalyst is carried on the heat exchange member.
 10. Theexhaust system according to claim 7, wherein the structure in which thefirst passage and the second passage are combined is placed in thevicinity of an exhaust manifold in the internal combustion engine. 11.The exhaust system according to claim 1, wherein the control memberserves to change each flow rate of the exhaust gas flowing into thefirst passage and the second passage.
 12. The exhaust system accordingto claim 11, wherein the control member comprises a valve that isoperated to close and open one of the first passage and the secondpassage at a predetermined degree.